This invention relates generally to maleimide compounds and methods of using the same as photoinitiators in photoactivatable polymerization systems.
Ethylenically unsaturated compounds, such as acrylate derivatives, can be polymerized by exposure to radiation, typically ultraviolet light, in the presence of a photoinitiating system. Typically, the photoinitiating system includes (1) a compound capable of initiating polymerization of the ethylenically unsaturated compound upon exposure to radiation (a xe2x80x9cphotoinitiatorxe2x80x9d) and optionally (2) a coinitiator or synergist, that is, a molecule which serves as a hydrogen atom donor. The coinitiators or synergists are typically alcohols, tertiary amines, amides, or ethers which have labile hydrogens attached to a carbon adjacent to a heteroatom.
Numerous photoinitiators with varying structures are commercially available for use in different systems. However, nearly all commercially available radiation curing processes require an initiator incorporated into the formulation, a large percent of which is not consumed. The use of conventional photoinitiators typically results in the production of small molecule photo-byproducts. The presence of the residual photo-active compounds and extractables can result degradation of the physical properties of the article, such as decreased light fastness, discoloration, and lower resistance to oxidative degradation. In addition, the residual photoinitiator can be extracted or leach out of the cured article or migrate to the surface of the article, which is undesirable in many applications.
Increasingly stringent environmental protection legislation has prompted the exploration and use of formulations which contain little or no volatile organic compounds (typically solvents). Thus the use of formulations with close to 100% reactive component is of great interest.
The present invention is directed to processes for radiation curing of photopolymerizable compounds using maleimides capable of initiating photopolymerization of radiation curable compounds, in place of conventional photoinitiators. In contrast to conventional photoinitiators, substantially all of the maleimide is consumed during initiation and photopolymerization. Thus, the processes of the invention can eliminate the problems associated with residual photoinitiator in the cured product, which are often observed when using conventional photoinitiators. The use of maleimides in accordance with the invention can also minimize the need for solvent based systems.
The maleimides can be useful as photoinitiators in the photopolymerization of ethylenically unsaturated compounds, and in particular, acrylate derivatives. The maleimides can also be useful as comonomers with polymerizable compounds.
Maleimides useful in the processes of the invention include at least one maleimide unit substituted at the nitrogen atom with a functionalized aliphatic radical. Exemplary maleimide compounds include maleimides of the formula (I) below: 
wherein:
(a) each R1 and R2 is independently selected from the group consisting of hydrogen, linear or branched C1 to C4 alkyl, and halogen;
(b) R is linear or branched C1 to C10 alkyl, heteroatom; or silicon; and
(c1) when R is C1 to C10 alkyl, FG is a functional group selected from the group consisting of xe2x80x94OR3, xe2x80x94SR3, xe2x80x94SiR3, xe2x80x94OC(O)N(R3)2, xe2x80x94OC(O)C(xe2x95x90CHR3)R3, xe2x80x94OC(O)R3, xe2x80x94C(O)R3, xe2x80x94N(R3)2, xe2x80x94C(O)OR3, xe2x80x94NCO, xe2x80x94C(O)N(R3)2, xe2x80x94OC(O)OR3, xe2x80x94CN, halogen, xe2x80x94CH2N-aryl-FG, xe2x80x94CH2N-aryl-R3xe2x80x94FG, sulfonic acid, quaternary ammonium, and salts thereof, with the proviso that when FG is xe2x80x94OR3, R is C1 to C4 linear or branched alkyl, and further in which each R3 is selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, arylalkyl, and alkylaryl, or
(c2) when R is a heterotom or silicon, FG is selected from the group consisting of hydrogen, alkyl, aryl, cycloalkyl, alkylaryl, arylalkyl, alkyl-FGxe2x80x2, and aryl-FGxe2x80x2, wherein FGxe2x80x2 is the same as FG as defined in (c1) above, or
FG is a functional group as defined in (c1) in combination with a spacer group linking said maleimide unit with at least one other maleimide unit to form a di- or multifunctional maleimide compound.
The present invention also provides novel maleimides and photopolymerizable compositions which include maleimide compounds comprising at least one maleimide unit of Formula (I) above as a component thereof.